The present invention concerns thermosetting powder coating compositions and methods for coating substrates with such compositions. In particular, the invention relates to thermosetting powder coating compositions resistant to overspray incompatibility defects, such as cratering or fuzziness, and use of such compositions.
Powder coating compositions have become increasingly important because they give off very little volatile material to the environment when cured. Typically, such emissions are limited to by-products of the curing reaction, such as blocking agents or volatile condensation products.
Powder coatings have found use as both decorative coatings and protective coatings. One commercial use of powder coating compositions is as an automotive primer, such as, for example, a primer applied to the lower panels of a vehicle for the purpose of increasing the resistance to damage from stone-chipping and other such impacts. This kind of primer is often based on polyester technology due to its impact strength. Such primers are usually overcoated with a topcoat such as a one-coat enamel or a basecoat/clearcoat topcoat system.
In addition to excellent chipping resistance, polyester-based powder primers have a large application range, typically 50 to 250 microns, over which they may be used to produce quality films. These compositions may be applied at the lower filmbuilds as a standard primer or at higher filmbuilds in areas where increased impact or stone-chipping resistance is desired. Polyester-based powder primers are also preferred for their excellent handling characteristics. Such powder coatings have excellent fluidizability and a wide application window (i.e. they may be applied under widely varying ambient conditions). In addition, polyester-based powder compositions have relatively long storage stabilities of over a year and are particularly resistant to impact fusion.
Powder coatings have also found use as blackout primers. Blackout primers are used on surfaces that will not be coated with a topcoat, such as the inside of the hood or trunk lid or on support pillars. Such primers also find use in two-tone applications. Because blackout primers will not be overcoated with a topcoat, they must have acceptable weathering characteristics. In other words, they must be resistant to chalking, loss of gloss, and other defects caused by exposure to the elements. Such blackout primers are usually based on acrylic technologies.
Although they have excellent weathering characteristics, acrylic powder coating compositions are less resistant to impact and stone-chipping damage when compared to polyester-based compositions. Acrylic powder primers are also more expensive, have poorer handling characteristics, have a much smaller application window, and poorer storage stability when compared to the polyester powder primers. However, their excellent exterior durability, superior smoothness and gloss, and better mar resistance makes them the preferred materials for many applications, such as the described use as blackout primers.
It would be desirable, then, to combine the use of both primers on articles where different areas require different types of protection. On an automobile or truck body, for example, the lower body can most advantageously be coated with the polyester-based powder primer for optimum chipping resistance, while the upper body areas and areas that will not be overcoated are optimally coated with an acrylic powder primer.
It would be advantageous to apply both types of primers in the same zone and cure both simultaneously. Application in the same zone and simultaneous curing provides an economical use of time and energy, while consecutive application and curing would add considerable expense in construction of a paint line, or may be burdensome or even impossible where it is necessary to work within the constraints of a manufacturing facility's existing line configuration. It has not been possible before now to commercialize such a dual system because overspray from the acrylic-based blackout primer caused unacceptable cratering in the areas coated with the uncured polyester-based primer. The usual additives that have been known in the art to control dirt cratering have been ineffective for preventing overspray cratering. Moreover, higher levels of traditional anti-cratering additives in the primer interfere with the adhesion of the topcoat or basecoat layer to the primer.
Surprisingly, we have discovered that the powder coatings of the invention, having both a very low molecular weight, crosslinkable acrylic resin component and a polyester resin component, do not exhibit any incompatibility with acrylic-based powder coatings applied on adjacent areas, such as produced unacceptable cratering or fuzziness in the film when conventional polyester-based powder primers were used. At the same time, the powder coating of the invention has excellent application and storage characteristics.